Downlink high rate packet transmission schemes (including HSDPA) in which a plurality of communication terminal apparatuses share a high rate and large capacity downlink channel are being developed. These transmission schemes employ scheduling techniques for improved transmission efficiency.
A scheduling technique refers to a technique of having a base station apparatus set the communication terminal apparatus for the transmission destination of packets and assign packets to transmit to the apparatus on a per time slot basis. Moreover, an adaptive modulation technique refers to a technique of determining modulation schemes or error correction coding schemes adaptively according to propagation path condition of the communication terminal apparatus to transmit packets to.
The base station apparatus estimates channel quality with each communication terminal apparatus on a per time slot basis in one frame, selects the communication terminal apparatus having the best channel quality, and assigns packets for the apparatus to each time slot. The base station apparatus performs error correction encoding and modulation of the packets by schemes determined through scheduling, and transmits the packets with information showing the scheduling result.
Based on the received information showing the scheduling result, each communication terminal apparatus performs decoding and error detection processing including CRC detection in a time slot to which a packet to its own station is assigned. When the packet data is correctly decoded, an ACK signal showing this result is transmitted to the base station apparatus, and, when the packet data is not correctly decoded, a NACK signal showing this result is transmitted to the base station apparatus.
Upon receiving the NACK signal the base station apparatus retransmits the previously transmitted packet or transmits a packet combinable with the previously transmitted packet in the time slot assigned by the scheduling. However, the packet is discarded and a new packet is transmitted when the number of times of retransmission reaches the maximum number of times for retransmission previously set in the system.
However, there is a problem with the conventional system that, the base station apparatus retransmits a packet upon receiving a NACK signal, and repeats packet retransmission to the communication terminal apparatus having poor channel quality when transmitting the packets to a plurality of communication terminal apparatuses during scheduling, thus overall system throughput is reduced.